This invention relates generally to building heating systems, and more particularly to control of heating systems.
As is known, in most building heating system applications, the heat exchanger (e.g., boiler or furnace), normally has excess reserve capacity to produce more heat into the heating fluid (water, if a boiler system; air, if a furnace system) than the system requires for heating the building. Accordingly, heating systems include "primary controls" which establish the maximum limit (called the "high limit") that the heat exchanger can run at before the control turns it off. Primary controllers work in cooperation with the building's thermostat(s), but override the thermostat if the high limit is attempted to be exceeded by a call for heat by the thermostat.
In typical heating systems when the thermostat determines that the building temperature has been achieved, the circulator (pump or fan, depending on the type of heating system) is turned off. Thus, the latent heat within the water or air is wasted. Such a loss is commonly referred to as a stand-by loss.
Add-on devices have been available for use in building heating systems to modulate the heat exchanger temperature as a function of the outdoor temperature. Such devices include an outdoor temperature sensor which is connected to the primary controller and to other portions of the heating system.
In U.S. Pat. No. 4,290,551 (Johnstone), there is disclosed an electronic controller for a heating system which includes means for modulating the boiler temperature in response to sensed outdoor temperature and for enabling the pump to circulate the heated water through the building even when the burner is off (providing that the water is above a predetermined temperature) to thus reduce stand-by loss. While the device of Johnstone appears generally suitable for its intended purposes, it does not include means for effecting various other control, display, warning and safety operations required for a fully integrated primary controller. Thus, many additional add-on devices are necessary for a complete system.
In heating systems having what is commonly referred to as a "summer/winter hookup" a domestic hot water coil is located within the boiler so that the domestic water flowing through the coil is heated by conduction from the boiler water. In such systems there must be some means provided to establish a low-limit for the boiler water temperature to insure that the domestic hot water is heated to the desired minimum level when the thermostat is not calling for heat. Since the temperature of the domestic hot water is typically desired in the range of 120.degree.-150.degree. F., the low-limit for boiler water temperature must be substantially higher, e.g., 150.degree.-190.degree. F. to effect sufficient heat transfer to the domestic hot water. Thus, in heating systems having a summer/winter hookup, devices must be provided for establishing the low-limit for the boiler. Such devices are frequently add-on units which must be physically incorporated into the heating system.
In systems utilizing a summer/winter hookup, add-on devices are available for providing a predetermined time period during which the burner is operated to produce domestic hot water in the event that there is no call for heat by the building's thermostat.
Heretofore, the most common type of primary control for a heating system has been an electromechanical control for sensing the heat exchanger temperature.
In such controllers used with either oil or gas-fired boilers, the boiler temperature sensing probe is in the form of a fluid-filled capillary tube. Such probes fail on occasion. Since the probe serves as the means for sensing if the boiler temperature is at the high-limit, the failure of the probe results in the controller calling for more heat (it thinks that the boiler temperature is too low), which action results in a runaway boiler, an obviously dangerous condition. Heretofore, prior art primary controllers have not provided sufficient means for preventing such runaway conditions.
In the interest of safety, and as required by Underwriters Laboratories, in oil-fired burner applications a flame detecting device is provided in the heating system to produce a signal indicative of the existence of a flame at the burner. The device is coupled to means for disabling ("locking-out") the burner in the event that a flame isn't sensed within a fixed period of time, e.g., 10 seconds after the building's thermostat calls for the burner to ignite. The most popular detecting device comprises a CdS cell located adjacent the burner head and electromechanical means coupled to the cell. The electromechanical means frequently comprises a bimetallic element which is heated to establish the time period during which flame ignition must occur. Due to the electromechanical construction of such flame sensing devices, the time constant established is not as precise as desired.
In gas-fired burner applications a CdS cell sensor is not necessary. In this regard, in such applications a thermocouple is normally included in the gas burner's pilot flame for providing a signal indicative of the presence of a pilot flame. The thermocouple is connected to means in the burner for precluding the main gas valve from opening in the event that the pilot flame is out.
Some display devices, e.g., meters, are available for displaying boiler temperature or other system conditions. However, such devices are merely add-on devices which must be physically incorporated into the system. Moreover, such display devices are not full-function displays for providing indications of various system conditions, e.g., CdS cell failure, primary probe failure, etc.